Web fs image links

To test:
 - Import images in develop
 - In the right panel, check the Download options... (down arrow) - should see "Image File" or "Image Files (zip)"
 - Clicking this should start the download (zipping first if multi-file). 
 - NB: Zipping multi-files can take a few secs with no feedback, but this behaviour already exists for dev_4_4. Probably a TODO for dev_4_4 and then develop.